Multilayer structures in which a gas barrier layer containing aluminum or aluminum oxide as a component is formed on a plastic film have been conventionally well-known. Such multilayer structures are used as packaging materials for protecting articles (such as foods) which are susceptible to quality change induced by oxygen. In many cases, such a gas barrier layer is formed on a plastic film by a dry process such as physical vapor deposition (PVD) or chemical vapor deposition (CVD).
For example, Patent Literature 1 discloses a composite structure having a gas barrier layer containing aluminum, the composite structure having a transparent gas barrier layer comprising a reaction product of aluminum oxide particles and a phosphorus compound. Patent Literature 1 discloses a method for forming the gas barrier layer, in which a coating liquid containing aluminum oxide particles and a phosphorus compound is applied onto a plastic film, then dried and heat-treated.
In an attempt to allow such a composite structure to maintain gas barrier properties at a high level when exposed to physical stresses, Patent Literature 2 discloses that a layer containing a polymer having a plurality of phosphorus atoms is stacked contiguous to a layer containing aluminum atoms.